Microsoft would answer questions about its targeting services only through a public relations spokesperson, who also asked that her name not be used. Microsoft's chief privacy officer, Brendon Lynch, did not respond to requests for comment.
Yahoo would not comment on specific clients, but said it has worked with Democrats and Republicans.
The Republican National Committee worked with both Yahoo and AOL to match Internet users to voter lists in 2007, according to Becki Donatelli, co-founder of Connell Donatelli, one of the most prominent Republican digital strategy firms. Contacted by ProPublica, AOL would not explain how its targeting service works.
The Republican National Committee also wouldn't provide details about its practices, but a spokeswoman said, "Targeting is one part of a larger playbook we have and will continue to employ. We follow legal guidelines and industry best practices." The Democratic National Committee did not respond to requests for comment.
When you see an online ad, you may assume it's comparable to a billboard 2014 identical to everyone who walks by. But that's not the case for many ads. In the milliseconds it takes for a page to load, advertisers can identify a particular user visiting a site, and choose ads to display based on what they know about that user.
For instance, surfers may be shown a shoe ad if they recently visited a shoe site. Most of this sort of targeting doesn't require your name. Political targeting does. Campaigns may want to reach only reliable party members, or independents who might swing their way.
In order to do this, campaigns assemble lists of names from public records of voters they hope to reach, using such criteria as party registration, turnout history and previous donations. The campaigns often hire companies that harvest vast amounts of consumer data about individual Americans, further refining their voter lists with factors not publicly available such as income, education, magazine subscriptions, and purchasing habits.
Finding voters online is difficult, since no public record connects voters to a particular Internet address.
That's where Microsoft, Yahoo, AOL and other lesser-known companies come in. Their enormous stores of registration data can serve as the bridge between particular Internet users and their voter information.
Online advertising is delivered with the help of cookies, tiny files that companies place on surfers' browsers. Cookies can be used to track people as they move from site to site, helping specialized firms most users have never heard of create detailed records of the sites users visit and the links they click. This tracking is typically done through anonymous ID numbers; the tracking firms and advertisers don't know you by name.
Microsoft and Yahoo's targeting service combines two crucial factors: their knowledge of users' personal information and their ability to add cookies to browsers. Over the years, Internet users have given these companies their name when they signed up for free programs like the Microsoft suite of services known as Windows Live, which includes Hotmail. (Microsoft said it does not sell campaigns access to information users provide when they register for Office or other Microsoft products they've bought.)